This invention relates to a dual-mode system and dual-mode wireless terminal in mobile communications. More particularly, the invention relates to a dual-mode wireless terminal that is capable of communicating in both a first mobile communication mode and a second mobile communication mode in a dual-mode environment, and to a dual-mode system for communicating using this terminal.
The present invention is used in a dual-mode environment in which a W-CDMA wireless system is made to coexist with the already existing wireless system GSM (Global System for Mobile Communication) or PDC (Personal Digital Cellular Telecommunication System) in Europe and Japan, etc.
In a dual-mode environment, a portable terminal uses either a W-CDMA or GSM (or PDC) wireless system (communication mode). In order to prepare for handover between base stations having different communication modes in such case, it is necessary for the terminal to ascertain whether there is a base station with both communication modes in the neighboring-cell. To achieve this, the terminal must periodically acquire information on base stations in the neighboring-cell even with regard to a communication mode that is not being used. At this time the terminal purposely starts up a communication unit conforming to a communication mode not in use and acquires information as to whether a base station in this unused communication mode exists in the neighboring-cell. The acquisition of information as to whether a base station in a communication mode currently being used is in the neighboring-cell is performed periodically.
FIG. 24 is a diagram for describing an example of operation according to the prior art. In a normal state shown at (A) of FIG. 24, a dual-mode terminal MS communicates with a W-CDMA base station BTS1 using a W-CDMA communication unit 1. The power supply of a GSM communication unit 2 is off at this time. However, GSM radio waves 4 are arriving from a GSM base station BTS2. Accordingly, a CPU (or DSP) 3 periodically issues a power-supply ON command PON to the GSM communication unit 2 and requests measurement of GSM radio waves (acquisition of neighboring-cell information), as indicated at (B) of FIG. 24). When neighboring-cell information is thus gathered, it is necessary to perform acquisition of neighboring-cell information that entails starting up a communication unit of a communication mode that is not in use and measuring the level of the radio waves.
In this mobile portable terminal MS, an important challenge concurrent with size reduction is how to extend continuous call operation and continuous standby time with a small battery. With the conventional method described above, the power supply of the communication unit of a communication mode not in use is turned on periodically and gathering of information concerning a neighboring base station of a communication not in use is performed periodically. That is, with the conventional method, it is necessary to periodically start up a communication unit of a communication mode other than the communication mode currently in use. Consequently, a commensurate amount of power is consumed, consumption of the terminal battery is hastened and continuous call time and standby time are shortened.
As an example of the prior art, there is a mobile telephone system (Patent Reference 1) in which when a mobile telephone system (PHS) currently in use leaves the service area, position registration in another mobile telephone system (GSM) is performed promptly. This involves retrieving and saving all control channels used by GSM immediately after the power supply of the terminal is turned on and executing position registration processing using a saved control channel when the terminal leaves the PHS area. Further, in a case where normal communication cannot be performed by any of the stored control channels, all control channels used by GSM are retrieved and saved. However, Patent Reference 1 does not relate to soft handover, in which communication is continued by changing over from a communicating base station to a base station of a different wireless system without instantaneous interruption. Further, Patent Reference 1 cannot search for a neighboring cell during travel in a dual-mode system in which handover is performed between base stations of different wireless systems. Patent Reference 1, therefore, does not solve the above-mentioned problems.    Patent Reference 1: Japanese Patent Application Laid-Open No. 11-298964